1. Field of the Invention
The present invention relates to methods and apparatuses for controlling the reaction of photoresist during semiconductor fabrication processes, and, more particularly, to controlling the reaction of chemically-amplified photoresist by analyzing the concentration of gas evolved during soft baking, exposure, and/or post-exposure baking steps.
2. Description of the Related Art
Photoresist materials have long enjoyed widespread use in the field of semiconductor fabrication. One form of photoresist technology utilized in conjunction with deep UV lithography is known as chemically-amplified resist (CAR).
Chemically-amplified photoresist systems are generally composed of 1) a polymer resin and 2) a photoacid generator (PAG), and may also include 3) a crosslinking agent, dye, or other additive.
CAR photoresist systems generally operate in the following manner. A nonactivated polymer resin is initially combined with a photoacid generator. Exposure of the photoacid generator to deep UV radiation changes the photoacid generator into a photoacid. The photoacid then triggers an activation reaction involving the polymer resin.
In the case of a positive CAR photoresist system, the photoacid catalyzes the deprotection of blocking groups on the polymer resin, increasing the polarity of the resin and hence, its solubility in aqueous base. Thus, the activation reaction for a positive CAR photoresist converts a nonpolar, insoluble polymer resin into a polar, soluble polymer resin.
In the case of a negative CAR photoresist system, the photoacid reacts with a cross-linking agent to trigger cross-linking between adjacent polymer chains. Thus, the activation reaction for a negative CAR photoresist system converts a single-chain, soluble polymer resin into a cross-linked, insoluble polymer resin.
FIG. 1 illustrates the sequence of process steps typically utilized in a CAR system. During the first spin-coat step, the multi-component photoresist mixture within a solvent carrier is dispensed onto a silicon wafer surface as the wafer is rapidly spun. The solvent carrier is typically propylene glycol monomethyl ether acetate (pgmea). The wafer is spun until the solvent carrier is substantially removed and the mixture dries as a film of uniform thickness over the entire surface.
The second photolithography step utilized in a CAR system is the soft bake. During soft baking, additional remaining solvent carrier is removed from the spun-dried photoresist mixture. The photoresist coated wafer is heated to approximately the glass-transition temperature T.sub.g of the nonactivated resin, such that excess solvent is driven off and the photoresist hardens.
The third step in the CAR process is exposure of the dried photoresist mixture to deep UV radiation (typically 193 nm or 248 nm). The exposure step requires utilization of complex photolithography equipment and precise masking techniques in order to ensure precise application of radiation only to those portions of photoresist intended to be exposed.
The fourth step in a deep UV photolithography process is the post-exposure bake (PEB). During the PEB, the photoresist is again subjected to temperatures on the order of T.sub.g. Thermal energy applied to the photoresist during the PEB causes the photoacid to diffuse within the resin. The diffusion of the photoacid fully activates the polymer resin in regions exposed to deep UV light, and also dampens any standing wave effects at the edges of exposed photoresist regions.
The fifth step associated with the CAR process-sequence is development. During development, an aqueous base solution is added to the exposed and baked photoresist, and a portion of the resin dissolves. Depending upon whether the CAR system is positive or negative, either activated or nonactivated polymer resin is dissolved in the aqueous base and removed.
After CAR photolithographic processing, the carefully created photoresist pattern serves to selectively mask etching of the silicon to form semiconducting structures having extremely precise shapes and sizes.
FIG. 2 illustrates the chemical reaction sequence of one exemplary positive CAR system. In the reaction sequence shown in FIG. 2, a photoacid generator consisting of triphenyl sulfonium salt 2 of trifluoroacetic acid is introduced into a mixture of nonpolar, aqueous base insoluble t-butoxycarbonyl (t-BOC)-substituted styrene 4 within a solvent consisting of pgmea. Exposing salt 2 to deep UV radiation causes disassociation of the triphenyl sulfonium to produce the photoacid trifluoroacetic acid 6.
Trifluoroacetic acid 6 then causes conversion of the nonpolar t-BOC group of styrene 4 into a polar hydroxy group. Subsequent heating of the mixture during a PEB step further enhances the rate of the conversion reaction. The resulting polyhydroxystyrene 8 is very polar.
Polyhydroxystyrene 8 present in photoresist regions exposed to deep UV radiation can be readily removed by dissolution in an aqueous base solution during the development step, to yield the desired pattern of photoresist.
Carbon dioxide 12 and isobutene 10 are side products of the conversion reaction of t-BOC substituted styrene 4 into polyhydroxystyrene 8. Both carbon dioxide 12 and isobutene 10 are volatile gases that diffuse into the air above the surface of the photoresist-coated wafer as activation reaction of the polymer resin proceeds to completion.
One limitation of current CAR processes is the difficulty in monitoring the amount of the solvent carrier and other materials driven from the spin coated wafer during the soft bake step. Overheating the photoresist mixture during the soft bake can cause degradation in the polymer resin or the PAG, hampering subsequent steps in the process.
On the other hand, underheating the photoresist mixture during the soft bake can result in excessive solvent remaining in the mixture. Excess solvent can cause problems during subsequent steps by altering the rate of generation of acid by the PAG, and/or by altering the rate of activation of the polymer by the acid.
Therefore, it is desirable to design methods and apparatuses that allow the extent of removal of solvent from a CAR coated photoresist wafer to be monitored and controlled during the soft bake step.
Another limitation in current CAR processes is that photoacid catalyzed conversion of the polymer resin cannot readily be monitored during the exposure or PEB steps.
Depending upon the CAR photoresist system utilized, activation of the polymer resin can occur predominantly either during the exposure step or the PEB step. Some CAR photoresist systems, such as the T-Boc protected polymer described above, are high activation energy systems. This means that even after H.sup.+ is generated by the PAG during exposure, additional thermal energy is required to initiate activation. This thermal energy is provided by the PEB step.
Other CAR photoresist systems, such as those utilizing acetal protection, are low activation energy systems. This means that once H.sup.+ is generated during exposure by the PAG, significant activation of the polymer resin takes place even in the absence of additional thermal energy.
With either high or low activation energy type systems, inability to ascertain the extent of activation of the polymer resin can create serious problems during subsequent development. For example, overexposing the wafer or overheating during PEB can result in excessive diffusion of the photoacid, causing the photoacid to migrate out of exposed regions of the photoresist into outlying, unexposed regions of the photoresist. Such excessive diffusion of the photoacid effectively degrades the resolution of the exposure. Excessive heating during PEB can also trigger unwanted cross-linking of the polymer resin, or deprotection of resin in locations not exposed to UV radiation. The above referenced phenomena create problems both in absolute terms, and in terms of maintaining consistency of critical dimension (CD) linewidth between different lots of wafers.
On the other hand, underexposing the wafer or underheating during PEB can produce excessively limited diffusion of the photoacid, causing inadequate activation of the polymer resin in exposed regions of the photoresist mixture. This can result in either too much or too little photoresist remaining after development, again adversely affecting the CD linewidth of etched features.
Therefore, it is desirable to design methods and apparatuses that allow activation of the polymer resin component of a CAR system to be monitored during the exposure and PEB steps.